1. Fields of the Invention
The present invention relates to an injection molding method for an injection molding machine and is particularly suitable for molding very thin molding products.
2. Description of the Related Art
Conventionally, there has been known an injection molding method for an injection molding machine suitable for molding thin products such as compact discs, as disclosed in Japanese Patent Application Laid-Open (kokai) No. 7 (1995)-256712. In the molding method, the advancement stop position of a screw is set based on an amount of resin to be injected and charged into a mold, and a mold clamping pressure is set for the mold. Further, a mold opening amount is set for the mold based on the amount of resin to be charged. During injection/charge operation, the screw is advanced to the advancement stop position and is stopped at the advancement stop position for a predetermined period of time by means of position control, while the preset mold clamping pressure is applied to the mold which has been opened by the above-mentioned mold opening amount. Thus, variation in the amount of resin charged into the cavity of the mold is eliminated, resulting in uniform weight of molding products.
Recently, even very thin molding products having a thickness of about 0.1 mm, such as diaphragms of speakers, have been molded from a synthetic resin by means of injection molding. When such thin products are molded, an injection pressure higher than that employed for ordinary molding products is applied to molten resin during an injection/charge operation. Further, since, as shown in FIG. 5, the ratio of the area of a thin molding product M to the thickness thereof is considerably large, when the cavity C of the mold 4 deforms slightly in the thicknesswise direction due to resin pressure, the deformation ratio of the molding product M becomes considerably large. In addition, the molten resin in the cavity C solidifies immediately after the injection/charge operation. Therefore, in reality, proper molding cannot be performed through use of the method disclosed in the patent publication, in which the mold is opened by a predetermined mold opening amount and a preset mold clamping pressure is applied to the mold after resin is injected and charged into the mold.
In order to solve this problem, there has been proposed an injection molding method in which molten resin in injected and charged under high pressure into the cavity C of a mold in an opened state, and immediately after completion of charge, application of pressure to the screw is stopped and the pressure within the cavity C is relieved. However, this method has the following drawbacks.
First, since molten resin injected and charged into the cavity C quickly solidifies as described above, the molten resin solidifies while the cavity C is in a deformed state if the pressure relief is not performed completely. Therefore, thin molding products of high precision and high quality cannot be obtained.
Second, since the above-described problem becomes more remarkable as the thickness of thin molding products decreases or the area thereof increases, the thickness and shape of the think molding products are limited, so that the types of moldable thin products are limited.